Nickel-base alloy heat treatments, nickel-base alloys, and articles including nickel-base alloys

ABSTRACT

A method for heat treating a 718-type nickel-base comprises heating a 718-type nickel-base alloy to a heat treating temperature, and holding the alloy at the heat treating temperature for a heat treating time sufficient to form an equilibrium or near-equilibrium concentration of δ-phase grain boundary precipitates within the nickel-base alloy and up to 25 percent by weight of total γ′-phase and γ″-phase. The 718-type nickel-base alloy is air cooled. The present disclosure also includes a 718-type nickel-base alloy comprising a near-equilibrium concentration of δ-phase grain boundary precipitates and up to 25 percent by weight of total γ′-phase and γ″-phase precipitates. Alloys according to the disclosure may be included in articles of manufacture such as, for example, face sheet, honeycomb core elements, and honeycomb panels for thermal protection systems for hypersonic flight vehicles and space vehicles.

BACKGROUND OF THE TECHNOLOGY

1. Field of the Technology

Embodiments of the present invention generally relate to methods of heattreating nickel-base alloys.

2. Description of the Background of the Technology

Alloy 718 (UNS 07718) is one of the most widely used nickel-base alloysand is described generally in U.S. Pat. No. 3,046,108, the specificationof which is hereby incorporated herein by reference in its entirety.Alloy 718 comprises elemental constituents within the ranges shown inthe following table, plus incidental impurities.

Element Ni Cr C Mn Si Mo Ni + Ta Ti Al B Fe weight % 50.0 17 to up to upto up to 2.8 4.75 0.65 0.20 up to rem. to 21.0 0.08 0.35 0.35 to to 5.5to to 0.006 55.0 3.3 1.15 0.8

The extensive use of Alloy 718 is at least partially attributable toseveral advantageous properties of the alloy. For example, Alloy 718 hashigh strength and stress-rupture properties up to about 1200° F. (648.9°C.). Additionally, Alloy 718 has good processing characteristics, suchas favorable castability and hot-workability, as well as goodweldability. These characteristics permit one to readily fabricatecomponents made from Alloy 718 and, when necessary, repair thosecomponents. As discussed below, several of Alloy 718's favorableproperties result from the alloy's precipitation-hardenedmicrostructure, which is predominantly strengthened by γ″-phaseprecipitates.

Precipitation-hardened, nickel-base alloys include two principalstrengthening phases: γ′-phase (or “gamma prime”) precipitates andγ″-phase (or “gamma double prime”) precipitates. Both the γ′-phase andthe γ″-phase are stoichiometric, nickel-rich intermetallic compounds.However, the γ′-phase typically comprises aluminum and titanium (i.e.,Ni₃(Al, Ti)) as the major alloying elements, while the γ″-phase includesprimarily niobium (i.e., Ni₃Nb). While both the γ′-phase and theγ″-phase form coherent precipitates in the face centered cubic austenitematrix, because the misfit strain energy associated with the γ″-phaseprecipitates (which have a body centered tetragonal crystal structure)is larger than that of the γ′-phase precipitates (which have a facecentered cubic crystal structure), γ″-phase precipitates tend to be moreefficient strengtheners than γ′-phase precipitates. That is, for thesame precipitate volume fraction and particle size, nickel-base alloysstrengthened primarily by γ″-phase precipitates are generally strongerthan nickel-base alloys strengthened primarily by γ′-phase precipitates.

One disadvantage of nickel-base alloys including a γ″-phase precipitatestrengthened microstructure is that the γ″-phase is unstable attemperatures higher than about 1200° F. (648.9° C.) and will transforminto the more stable δ-phase (or “delta-phase”). While δ-phaseprecipitates have the same composition as γ″-phase precipitates (i.e.,Ni₃Nb), δ-phase precipitates have an orthorhombic crystal structure andare incoherent with the austenite matrix. Accordingly, the strengtheningeffect of δ-phase precipitates on the matrix is generally considered tobe negligible. Therefore, a result of the transformation to δ-phase isthat certain mechanical properties of Alloy 718, such as stress-rupturelife, deteriorate rapidly at temperatures above about 1200° F. (648.9°C.). Therefore, the use of Alloy 718 typically has been limited toapplications in which the alloy is subjected to temperatures below 1200°F. (648.9° C.).

In order to form the desired precipitation-hardened microstructure,nickel-base alloys are subjected to a heat treatment or precipitationhardening process. The precipitation hardening process for a nickel-basealloy generally involves solution treating the alloy by heating thealloy at a temperature sufficient to dissolve substantially all ofγ′-phase and γ″-phase precipitates in the alloy (i.e., a temperaturenear, at, or above the solvus temperature of the precipitates), coolingthe alloy from the solution treating temperature, and subsequently agingthe alloy in one or more aging steps. Aging is conducted at temperaturesbelow the solvus temperature of the gamma precipitates in order topermit the desired precipitates to develop in a controlled manner.

The development of the desired microstructure in the nickel-base alloydepends upon both the alloy composition and the precipitation hardeningprocess (i.e., the solution treating and aging processes) employed. Forexample, a typical precipitation hardening procedure for Alloy 718 forhigh temperature service involves solution treating the alloy at atemperature of 1750° F. (954.4° C.) for 1 to 2 hours, air cooling thealloy, followed by aging the alloy in a two-step aging process. Thefirst aging step involves heating the alloy at a first aging temperatureof 1325° F. (718.3° C.) for 8 hours, cooling the alloy at about 50 to100° F. per hour (28 to 55.6° C. per hour) to a second aging temperatureof 1150° F. (621.2° C.), and aging the alloy at the second agingtemperature for 8 hours. Thereafter, the alloy is air cooled to roomtemperature. The precipitation-hardened microstructure that resultsafter the above-described heat treatment is comprised of discreteγ′-phase and γ″-phase precipitates, but is predominantly strengthened bythe γ″-phase precipitates with minor amounts of the γ′-phaseprecipitates playing a secondary strengthening role.

In an effort to increase the allowable service temperatures ofnickel-base alloys, several γ′-phase strengthened nickel-base alloyshave been developed. An example of such an alloy is Waspaloy nickel-basealloy (UNS N07001), which is commercially available as ATI Waspaloyalloy from ATI Allvac, Monroe, N.C. USA. Because Waspaloy nickel-basealloy includes higher levels of alloying additions, including nickel,cobalt, and molybdenum, than Alloy 718, Waspaloy alloy typically is morecostly than Alloy 718. Also, because of the faster precipitationkinetics of γ′-phase precipitates relative to γ″-phase precipitates, thehot workability and weldability of Waspaloy alloy is generallyconsidered to be inferior to Alloy 718.

Another γ′-phase strengthened nickel-base alloy is ATI 718Plus® alloy,which is commercially available from ATI Allvac, Monroe, N.C. ATI718Plus® alloy is disclosed in U.S. Pat. No. 6,730,264 (“the U.S. '264patent”), which hereby is incorporated herein by reference in itsentirety. A feature of ATI 718Plus® alloy is that the alloy's aluminum,titanium and/or niobium levels and their relative ratio are adjusted ina manner that provides a thermally stable microstructure andadvantageous high-temperature mechanical properties, includingsubstantial rupture and creep strength. The aluminum and titaniumcontents of ATI 718Plus® alloy, in conjunction with niobium content,results in the alloy being strengthened by γ′-phase and γ″-phase, withγ′-phase being the predominant strengthening phase. Unlike therelatively high titanium/low aluminum composition typical of certainother nickel-base superalloys, the composition of ATI 718Plus® alloy hasa relatively high ratio of atomic percent aluminum to atomic percenttitanium that is believed to increase thermal stability. The thermalstability characteristics of ATI 718Plus® alloy are important formaintaining good mechanical properties, such as stress ruptureproperties, after long periods of exposure to high temperatures.

ATI 718Plus® alloy can be subjected to processing including solutionannealing, cooling, and aging. A typical heat treatment for ATI 718Plus®alloy is depicted in FIG. 1 as a schematic representation of atime-temperature heat treatment profile. A typical heat treatment forATI 718Plus® alloy includes a solution treatment at temperatures between1750° F. (954.4° C.) and 1800° F. (982.2° C.) to dissolve any γ′-phaseand γ″-phase and precipitate a small amount of δ-phase. The amount ofδ-phase precipitated is typically less than about half the lowtemperature equilibrium content. The solution treatment is followed byaging at 1450° F. (787.8° C.) for 2 to 8 hours, and then at 1300° F.(704.4° C.) for an additional 8 hours to precipitate coherent γ′-phaseparticles. The alloy may be further processed to an article ofmanufacture or into any other desired form.

Additional heat treatments for strengthening ATI 718Plus® alloy aredisclosed in U.S. Pat. Nos. 7,156,932; 7,491,275; and 7,527,702, each ofwhich is hereby incorporated herein by reference in its entirety. U.S.Pat. No. 7,531,054 (the “U.S. '054 patent”) discloses a heat treatmentfor ATI 718Plus® alloy that includes direct aging. In the process of theU.S. '054 patent, after hot working the ATI 718Plus® alloy, the alloy israpidly and directly cooled to an aging temperature of about 1400° F.(760° C.) to prevent the precipitation of coarse γ′-phase precipitates.The cooled alloy is aged at the aging temperature or is further cooledto room temperature.

In general, precipitation hardened alloys are not designated for useabove their age hardening temperatures. Precipitation hardened nickelalloys have not been used in applications where the alloy may experiencethermal cycling, where the alloys may be repeatedly exposed totemperatures above their age hardening temperatures and then cooled totemperatures below their age hardening temperatures. Conventional agehardening practices for nickel-base alloys, as summarized above, wouldnot result in consistent mechanical properties over the service periodfor nickel-base alloys that would be exposed to thermal cycling in whichtemperatures exceed the alloy's age hardening temperature.

It would be desirable to provide a heat treatment for precipitationhardened nickel-base alloys that provides a robust microstructure andimparts properties that are not significantly affected by thermalcycling. A nickel-base alloy treated in this way may be advantageous foruse in, for example, face sheet and honeycomb core of thermal protectionsystems for hypersonic flight vehicles, and as a material in otherarticles of manufacture that experience in-service thermal cycling.

SUMMARY

According to one aspect of the present disclosure, a method for heattreating a 718-type nickel-base alloy comprises heating a 718-typenickel-base alloy to a heat treating temperature, and holding the718-type nickel-base alloy at the heat treating temperature for a heattreating time sufficient to form an equilibrium or near-equilibriumconcentration of δ-phase grain boundary precipitates within thenickel-base alloy. The heat treating results in the formation of up to25 percent by weight of total γ′-phase and γ″-phase within thenickel-base alloy. After holding the 718-type alloy at the heat treatingtemperature for the heat treating time, the 718-type nickel-base alloyis cooled and retains the δ-phase grain boundary precipitates in thealloy.

According to another aspect of the present disclosure, a method of heattreating a nickel-base alloy comprises heating the nickel-base alloy toa heat treating temperature in a heat treating temperature range of atemperature that is 20° F. greater than the nose of theTime-Temperature-Transformation diagram (“TTT diagram”) for delta phaseprecipitation up to 100° F. (55.6° C.) below the nose of the TTTdiagram, and holding the nickel-base alloy within the heat treatingtemperature range for a heat treating time in a range of 30 minutes to300 minutes. After holding the nickel-base alloy within the heattreating temperature range for the heat treating time, the nickel-basealloy is air cooled to ambient temperature. In a non-limitingembodiment, the nickel-base alloy is cooled at a cooling rate no greaterthan 1° F. per minute (0.56° C. per minute).

In a non-limiting embodiment, the nickel-base alloy comprises, inpercent by weight, 0.01 to 0.05 carbon, up to 0.35 manganese, up to0.035 silicon, 0.004 to 0.020 phosphorus, up to 0.025 sulfur, 17.00 to21.00 chromium, 2.50 up to 3.10 molybdenum, 5.20 up to 5.80 niobium,0.50 up to 1.00 titanium, 1.20 to 1.70 aluminum, 8.00 to 10.00 cobalt,8.00 to 10.00 iron, 0.008 to 1.40 tungsten, 0.003 to 0.008 boron,nickel, and incidental impurities.

According to an additional aspect of the present disclosure, a 718-typenickel-base alloy is provided comprising nickel, chromium, and iron. Thenickel-base alloy is strengthened by niobium and, optionally one or moreof aluminum and titanium alloying additions, and the alloy comprises anaustenite matrix including austenite grain boundaries. An equilibrium ornear-equilibrium concentration of δ-phase precipitates exists at theaustenite grain boundaries in the 718-type alloy, and the alloy includesup to 25 percent by weight of γ′-phase and γ″ precipitates.

According to a further aspect of the present disclosure, a process formaking an article of manufacture includes at least one of the methodsdisclosed herein. In certain non-limiting embodiments, the process maybe adapted for making an article of manufacture selected from a facesheet, a honeycomb core, and a honeycomb panel of a thermal protectionsystem for a hypersonic flight vehicle.

According to yet another aspect of the present disclosure, an article ofmanufacture comprises an alloy disclosed herein. Such an article ofmanufacture may be selected from, but is not limited to, a face sheet, ahoneycomb core, and a honeycomb panel of a thermal protection system fora hypersonic flight vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of alloys and methods described herein maybe better understood by reference to the accompanying drawings in which:

FIG. 1 is temperature-time heat treatment diagram of a conventionalprior art heat treatment for strengthening nickel-base alloys;

FIG. 2 is a schematic representation of one example of a metallicthermal protection system;

FIG. 3A is a schematic representation of one example of a honeycombpanel;

FIG. 3B is a schematic representation of an exploded view of one exampleof a honeycomb panel;

FIG. 4 is a flow diagram of a non-limiting embodiment of a heattreatment for a nickel-base alloy according to the present disclosure;

FIG. 5A is a Time-Temperature-Transformation curve for Alloy 718nickel-base superalloy;

FIG. 5B is a Time-Temperature-Transformation curve for ATI 718Plus®alloy;

FIG. 6 is a schematic temperature-time plot for a non-limitingembodiment of a method according to the present disclosure for heattreating a nickel-base alloy;

FIG. 7 is a schematic representation of thermal cycling used to evaluatenon-limiting embodiments of methods of heat treating nickel-base alloysaccording to the present disclosure;

FIG. 8 provides plots of ultimate tensile strength as a function ofnumber of thermal cycles for ATI 718Plus® alloy treated withnon-limiting heat treating methods according to the present disclosure,and compared with conventional γ′/γ″ heat treating methods before andafter thermal cycling to 1650° F. (898.9° C.) and 1550° F. (843.3° C.);

FIG. 9 provides plots of relative retained ultimate tensile strength asa function of number of thermal cycles for ATI 718Plus® alloy treatedwith non-limiting heat treating methods according to the presentdisclosure, and compared with conventional γ′/γ″ heat treating methodsbefore and after thermal cycling to 1650° F. (898.9° C.) and 1550° F.(843.3° C.);

FIG. 10 provides plots of yield strength as a function of number ofthermal cycles for ATI 718Plus® alloy treated with non-limiting heattreating methods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.) and 1550° F. (843.3° C.);

FIG. 11 includes plots of relative retained yield strength as a functionof number of thermal cycles for ATI 718Plus® alloy treated withnon-limiting heat treating methods according to the present disclosure,and compared with conventional γ′/γ″ heat treating methods before andafter thermal cycling to 1650° F. (898.9° C.) and 1550° F. (843.3° C.);

FIG. 12 includes plots of percent elongation as a function of number ofthermal cycles for ATI 718Plus® alloy treated with non-limiting heattreating methods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.) and 1550° F. (843.3° C.);

FIG. 13 includes plots of relative percent elongation as a function ofnumber of thermal cycles for ATI 718Plus® alloy treated withnon-limiting heat treating methods according to the present disclosure,and compared with conventional γ′/γ″ heat treating methods before andafter thermal cycling to 1650° F. (898.9° C.) and 1550° F. (843.3° C.);

FIG. 14 includes plots of ultimate tensile strength as a function ofnumber of thermal cycles for Alloy 718 treated with non-limiting heattreating methods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.);

FIG. 15 includes plots of relative retained ultimate tensile strength asa function of number of thermal cycles for Alloy 718 treated withnon-limiting heat treating methods according to the present disclosure,and compared with conventional γ′/γ″ heat treating methods before andafter thermal cycling to 1650° F. (898.9° C.);

FIG. 16 includes plots of yield strength as a function of number ofthermal cycles for Alloy 718 treated with non-limiting heat treatingmethods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.);

FIG. 17 includes plots of relative retained yield strength as a functionof number of thermal cycles for Alloy 718 treated with non-limiting heattreating methods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.);

FIG. 18 includes plots of percent elongation as a function of number ofthermal cycles for Alloy 718 treated with non-limiting heat treatingmethods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.);

FIG. 19 includes plots of relative percent elongation as a function ofnumber of thermal cycles for Alloy 718 treated with non-limiting heattreating methods according to the present disclosure, and compared withconventional γ′/γ″ heat treating methods before and after thermalcycling to 1650° F. (898.9° C.);

FIG. 20A is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a non-limitingembodiment of the present disclosure;

FIG. 20B is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a non-limitingembodiment of the present disclosure after 5 thermal cycles from ambienttemperature to 1650° F. (898.9° C.) and back to ambient temperature;

FIG. 20C is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a conventionalγ′/γ″ heat treatment;

FIG. 20D is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a conventionalγ′/γ″ heat treatment after 5 thermal cycles from ambient temperature to1650° F. (898.9° C.) and back to ambient temperature;

FIG. 21A is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a non-limitingembodiment of the present disclosure;

FIG. 21B is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a non-limitingembodiment of the present disclosure after 5 thermal cycles from ambienttemperature to 1550° F. (843.3° C.) and back to ambient temperature;

FIG. 21C is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a conventionalγ′/γ″ heat treatment;

FIG. 21D is a dark field optical micrograph of a surface region of asheet of ATI 718Plus® alloy heat treated according to a conventionalγ′/γ″ heat treatment after 5 thermal cycles from ambient temperature to1550° F. (843.3° C.) and back to ambient temperature;

FIG. 22A is a dark field optical micrograph of a surface region of asheet of Alloy 718 heat treated according to a non-limiting embodimentof the present disclosure;

FIG. 22B is a dark field optical micrograph of a surface region of asheet of Alloy 718 heat treated according to a non-limiting embodimentof the present disclosure after 5 thermal cycles from ambienttemperature to 1650° F. (898.9° C.) and back to ambient temperature;

FIG. 22C is a dark field optical micrograph of a surface region of asheet of Alloy 718 heat treated according to a conventional γ′/γ″ heattreatment; and

FIG. 22D is a dark field optical micrograph of a surface region of asheet of Alloy 718 heat treated according to a conventional γ′/γ″ heattreatment after 5 thermal cycles from ambient temperature to 1650° F.(898.9° C.) and back to ambient temperature.

The reader will appreciate the foregoing details, as well as others,upon considering the following detailed description of certainnon-limiting embodiments according to the present disclosure.

DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

In the present description of non-limiting embodiments, other than inthe operating examples or where otherwise indicated, all numbersexpressing quantities or characteristics are to be understood as beingmodified in all instances by the term “about”. At the very least, andnot as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques.

Any patent, publication, or other disclosure material that is said to beincorporated, in whole or in part, by reference herein is incorporatedherein only to the extent that the incorporated material does notconflict with existing definitions, statements, or other disclosurematerial set forth in the present disclosure. As such, and to the extentnecessary, the disclosure as set forth herein supersedes any conflictingmaterial incorporated herein by reference. Any material, or portionthereof, that is said to be incorporated by reference herein, but whichconflicts with existing definitions, statements, or other disclosurematerial set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the existingdisclosure material.

Certain nickel-base alloys are being considered for use as face sheetand core elements for honeycomb panels that will be used in thermalprotection systems for hypersonic flight vehicles. The surfacetemperature of a hypersonic flight vehicle when in service will cyclebetween ground temperature and about 2200° F. (1204° C.) at least onceper flight mission. Exposure of age hardened nickel-base alloys to sucha thermal cycle may result in a change in the volume fraction and sizeof precipitate phases, particularly the γ′-phase and γ″-phaseprecipitates, as compared with the as-brazed and age-hardened conditionof the nickel-base alloy prior to the first flight mission flown.Further, it is to be expected that different flight missions will havedifferent thermal exposure profiles, resulting in a microstructure andmechanical properties of the age hardened nickel-base alloy that willvary based on the mission or missions flown.

A thermal protection system (TPS) protects key components of hypersonicflight vehicles and spacecraft from melting or being otherwise damagedfrom the heat generated at high speeds and/or during re-entry into theatmosphere. A TPS must be lightweight, reusable, and maintainable. Aschematic representation of one example of a metallic TPS (10) employinghoneycomb panels is presented in FIG. 2. The metallic TPS (10) may befastened to an external reinforcing member (12) of a component such as,for example, a cryogenic fuel tank (not shown) of a hypersonic flightvehicle or space vehicle. The metallic TPS (10) may comprise, forexample, metallic honeycomb panels (14) and foil encapsulated insulation(16).

One example of a honeycomb panel (20) is schematically depicted in FIG.3A, and an exploded schematic view of honeycomb panel (20) is depictedin FIG. 3B. Honeycomb panel (20) comprises a compartmentalized honeycombcore (22) interposed between and joined to opposing face sheets (24),thereby providing multiple enclosed chambers within the panel. As usedherein, the term “honeycomb panel” refers to a metallic honeycomb coreinterposed or sandwiched between metallic face sheets. As used herein,the terms “honeycomb” and “honeycomb core” refer to a manufacturedproduct comprising an arrangement of generally polygonal-shaped (e.g.,hexagonal-shaped) cells formed from alloy foil and which may be appliedas core material interposed or sandwiched between two face sheets of ametallic material or other suitable material to provide a honeycombpanel. As used herein, the term “face sheet” refers to metallic foil,sheet, or plate that is joined to a metallic honeycomb core as generallydepicted in FIG. 2 to provide a honeycomb panel. Honeycomb cores areused to form honeycomb panels by adhesively bonding, brazing, welding,or otherwise joining face sheets to the open cells of the honeycombcore. A honeycomb panel exhibits high compression and shear properties,while minimizing the weight required to achieve these propertiescompared with a monolithic material. Honeycomb panels are used inaerospace, marine, and ground transportation applications in order toreduce vehicle weight and reduce fuel consumption. Methods of forminghoneycomb core, face sheets, and honeycomb panels are well known topersons skilled in the art and, thus, are not further described herein.

It is believed that the aerospace industry has only seriously consideredthe use of metallic TPSs within the past 15 years, and little attentionhas been given to alloys used for the face sheet and honeycomb core ofaerospace panels. Generally, the precipitation hardened alloys have beenavoided, and solution strengthened or oxide dispersion strengthenedalloys have been used in TPS applications because of the inherent phaseinstability of a precipitation hardened alloy microstructure.

Certain non-limiting embodiments of the present invention are directedto methods of heat treating nickel-base alloys to provide amicrostructure that is generally stable when subjected to thermalcycling. Because the microstructure achieved by the present methodsremains substantially the same during the one or more thermal cycles towhich the nickel-base alloy is subjected, the mechanical properties ofthe nickel-base alloy will remain substantially the same at a particulartemperature when the alloy is thermally cycled back to that particulartemperature. For example, non-limiting embodiments of heat treatingmethods according to the present disclosure provide a nickel-base alloywith certain properties at 1550° F. (843.3° C.) in a second thermalcycle that are substantially the same as the properties of the samenickel-base alloy at 1550° F. (843.3° C.) in a tenth thermal cycle, butwhich are not the same as the mechanical properties of the nickel-basealloy at, for example, 1650° F. (898.9° C.) or at 1700° F. (926.7° C.).

It was determined that γ′-phase contributes little to the strength oflow γ′-phase volume fraction alloys such as, for example, Alloy 718, attemperatures above about 1500° F. (815.6° C.). Therefore, it wasdetermined that heat treatments designed to optimize γ′-phase are notbeneficial for applications such as hypersonic flight vehicle TPSs,which may experience repeated thermal cycling between ambienttemperature and temperatures up to 2200° F. (1204° C.). Heat treatmentsthat provide a stable microstructure during such thermal cycling wouldbe beneficial for use in thermal protection systems.

For example, a non-limiting embodiment according to the presentdisclosure is directed to a method of heat treating a nickel-base alloyto produce a thermally stable microstructure in a 718-type nickel-basealloy that is able to withstand thermal cycling between ambient groundtemperatures and a maximum temperature of about 1450° F. (787.8° C.) toabout 75° F. (42° C.) below the δ-solvus temperature. The thermallystable microstructure is a microstructure that provides the alloy withmechanical properties that do not substantially change when exposed tothermal cycles in a temperature range between ambient temperature and amaximum temperature in a range of about 1450° F. (787.8° C.) to about75° F. (42° C.) below the δ-solvus temperature of the alloy. Ifin-service thermal cycling results in the exposure of the nickel-basealloy to temperatures above the heat treating temperature rangeaccording to the present disclosure, detrimental changes to the alloy'smicrostructure and mechanical properties may occur.

The δ-solvus temperature for Alloy 718 is about 1881° F. (1027° C.). The5-solvus temperature for ATI 718Plus® alloy is about 1840° F. (1004°C.). The δ-solvus temperatures of other nickel-base alloys are known orcan be readily determined without undue experimentation by a personhaving ordinary skill in the metallurgical arts.

In a non-limiting embodiment according to this disclosure, the methodresults in an equilibrium or near-equilibrium concentration of grainboundary δ-phase at the grain boundaries of the austenite matrix, withprecipitation of up to 25 percent by weight of total γ′-phase andγ″-phase precipitates. Given the precipitation of an equilibrium ornear-equilibrium concentration of grain boundary δ-phase in embodimentsaccording to this disclosure, embodiments of the heat treating methodsaccording to this disclosure are referred to herein as “δ-phase heattreatments”.

Embodiments of the δ-phase heat treatments according to the presentdisclosure provide a volume fraction of δ-phase that does notsubstantially decrease until in-service temperatures exceed about 75° F.(42° C.) below the δ-solvus temperature. Therefore, embodiments of theδ-phase heat treatments disclosed herein promote a stable microstructurefor applications in which temperatures may cycle up to a maximumtemperature of about 75° F. (42° C.) below the δ-solvus temperature. Theδ-phase precipitated at the grain boundaries according to methods of thepresent disclosure also serves the purpose of preventing grain growth,further stabilizing the microstructure. Embodiments of the δ-phase heattreatments disclosed herein result in lower strengths in nickel-basealloys below about 1500° F. (815.6° C.). However, in comparison, whilein service, a conventionally heat treated 718-type nickel-base alloypart subjected to temperatures above 1500° F. (815.6° C.) would onlyexhibit relatively higher strength at temperatures below 1500° F.(815.6° C.) for the first thermal cycle to which the part is subjected.

Although not limiting herein, embodiments of the δ-phase heat treatmentsdisclosed herein can be used in conjunction with nickel-base alloycompositions containing niobium (Nb), including 718-type nickel-basealloys and derivatives thereof. As used herein, the term “nickel-basealloy” refers to an alloy including predominantly nickel, along with oneor more other alloying elements and incidental impurities. As usedherein, the term “718-type nickel-base alloy” means a nickel-base alloy,as defined herein, comprising or consisting of nickel, chromium, iron,strengthening additions of niobium, and optionally one or both ofaluminum and titanium, along with incidental impurities. Non-limitingexamples of 718-type nickel-base alloys include Alloy 718 and otheralloys discussed hereinbelow.

A non-limiting example of a 718-type nickel-base alloy for whichnon-limiting embodiments of heat treatments according to the presentdisclosure are believed to be particularly well suited is a nickel-basealloy including nickel, chromium, up to 14 weight percent iron,strengthening additions of niobium, optionally one or both of aluminumand titanium alloying additions, and incidental impurities. Anothernon-limiting example of a 718-type nickel-base alloy for whichnon-limiting embodiments of heat treatments according to the presentdisclosure are believed to be particularly well suited is a nickel-basealloy, as defined herein, including chromium, 6 up to 14 weight percentiron, strengthening additions of niobium, optionally one or more ofaluminum and titanium alloying additions, and incidental impurities.

An additional non-limiting example of a 718-type nickel-base alloys withwhich embodiments of heat treating methods according to the presentdisclosure may be used is the nickel-base alloy disclosed in U.S. Pat.No. 6,730,264 (“the '264 patent”), which comprises or consists of, inpercent by weight: up to 0.1 carbon; 12 to 20 chromium; up to 4molybdenum; up to 6 tungsten; 5 to 12 cobalt; 6 to 14 iron; 4 to 8niobium; 0.6 to 2.6 aluminum; 0.4 to 1.4 titanium; 0.003 to 0.03phosphorus; 0.003 to 0.015 boron; nickel; and incidental impurities;wherein a sum of the weight percent of molybdenum and the weight percentof tungsten is at least 2 and not more than 8; wherein a sum of atomicpercent aluminum and atomic percent titanium is from 2 to 6; wherein aratio of atomic percent aluminum to atomic percent titanium is at least1.5; and wherein the sum of atomic percent aluminum and atomic percenttitanium divided by atomic percent niobium is from 0.8 to 1.3. Theentire disclosure of U.S. Pat. No. 6,730,264 is hereby incorporated byreference herein.

Still another non-limiting example of a 718-type nickel-base alloy withwhich embodiments of heat treating methods according to the presentdisclosure may be used is a nickel-base alloy disclosed in the U.S. '264patent and which comprises or consists of, in percent by weight: 50 to55 nickel; 17 to 21 chromium; 2.8 to 3.3 molybdenum; 4.7 percent to 5.5niobium; up to 1 cobalt; 0.003 to 0.015 boron; up to 0.3 copper; up to0.08 carbon; up to 0.35 manganese; 0.003 to 0.03 phosphorous; up to0.015 sulfur; up to 0.35 silicon; iron; aluminum; titanium; andincidental impurities; wherein the sum of atomic percent aluminum andatomic percent titanium is from about 2 to about 6 atomic percent;wherein the ratio of atomic percent aluminum to atomic percent titaniumis at least about 1.5; and wherein the sum of atomic percent of aluminumplus atomic percent of titanium divided by atomic percent of niobium isfrom about 0.8 to about 1.3. In certain embodiments of the alloy, theweight percent of iron is from 12 up to 20.

Yet another non-limiting example of a 718-type nickel-base alloy withwhich embodiments of heat treating methods according to the presentdisclosure may be used is ATI 718Plus® alloy (UNS N07818), which is anickel-base alloy that is available from ATI Allvac, Monroe, N.C., USA,and that comprises or consists of, in percent by weight: 17.00 to 21.00chromium; 2.50 to 3.10 molybdenum; 5.20 to 5.80 niobium; 0.50 to 1.00titanium; 1.20 to 1.70 aluminum; 8.00 to 10.00 cobalt; 8.00 to 10.00iron; 0.008 to 1.40 tungsten; 0.003 to 0.008 boron; 0.01 to 0.05 carbon;up to 0.35 manganese; up to 0.035 silicon; 0.004 to 0.020 phosphorus; upto 0.025 sulfur; nickel; and incidental impurities. AMS 5441 and AMS5442, which relate to corrosion and heat-resistant bars, forgings, andrings, are two AMS specifications describing heat treatmentsconventionally used with ATI 718Plus® alloy. Each of AMS 5441 and AMS5442 is hereby incorporated by reference herein in its entirety.

Still another non-limiting example of a 718-type nickel-base alloy withwhich embodiments of heat treating methods according to the presentdisclosure may be used is Alloy 718 (UNS N07718), the composition ofwhich is well known in the industry. In certain non-limitingembodiments, Alloy 718 comprises or consists of, in percent by weight:50.0 to 55.0 nickel; 17 to 21.0 chromium; up to 0.08 carbon; up to 0.35manganese; up to 0.35 weight percent silicon; 2.8 to 3.3 molybdenum;greater than zero up to 5.5 niobium and tantalum, wherein the sum ofniobium and tantalum is 4.75 to 5.5; 0.65 to 1.15 titanium; 0.20 to 0.8aluminum; up to 0.006 boron; iron; and incidental impurities.

As used herein, the term “mechanical properties” refers to properties ofan alloy relating to the elastic or inelastic reaction when force isapplied to the alloy, or that involve the relationship between stressand strain that results when force is applied to the alloy. Mechanicalproperties, within the meaning of the present disclosure, specificallyrefer to tensile strength, yield strength, elongation, andstress-rupture life. As used herein, the term “thermally stablemechanical properties” refers to a condition wherein mechanicalproperties of an alloy do not change by more than 20% when the alloy issubjected to repeated thermal cycling between ambient ground temperatureand 75° F. (41.7° C.) below the δ-solvus temperature. As used herein,the term “ambient ground temperature” is defined as any temperature ofthe surroundings resulting from a natural terrestrial climate at groundlevel.

The present inventors have noted an impact of the thermal cycle peaktemperature on the degree of deterioration of mechanical properties fornickel-base alloys for a given δ-phase heat treatment according tonon-limiting embodiments of the present disclosure. The choice of theδ-phase heat treating temperature should be chosen to match or closelymatch the expected peak in-service temperature of the nickel-base alloy.

Referring now to FIG. 4, in a non-limiting embodiment according to thepresent disclosure, a method for δ-phase heat treating a 718-typenickel-base alloy (30) comprises: heating (32) a 718-type nickel-basealloy to a heat treating temperature in a heat treating temperaturerange; holding (34) the nickel-base alloy within the heat treatingtemperature range for a heat treating time that is sufficient to form anequilibrium or near-equilibrium concentration of δ-phase grain boundaryprecipitates within the nickel-base alloy, and also up to 25 percent byweight of total γ′-phase and γ″-phase within the nickel-base alloy; andair cooling (36) the 718-type nickel-base alloy.

As used herein, the term “heat treating temperature” is defined as atemperature that results in precipitation of an equilibrium ornear-equilibrium concentration of δ-phase precipitates at the grainboundary of a 718-type nickel-base alloy and up to 25 percent by weightof total γ′-phase and γ″-phase. As used herein, the term “heat treatingtime” means a time sufficient to precipitate an equilibrium ornear-equilibrium concentration of δ-phase precipitates at the grainboundaries of a 718-type nickel-base alloy and up to 25 percent byweight of total γ′-phase and γ″-phase. As used herein, the term“equilibrium concentration” is defined as the maximum concentration ofδ-phase precipitates that can form at the heat treating temperatureaccording to the composition of the nickel-base alloy or 718-typenickel-base alloy. As used herein, the term “near-equilibriumconcentration” means the condition wherein a nickel-base alloy includesabout 5 percent to about 35 percent by weight of δ-phase at the grainboundaries. In a non-limiting embodiment, after a δ-phase heattreatment, the nickel-base alloy may include about 6 percent to about 12percent by weight of δ-phase precipitated at the grain boundaries. Sucha result is observed to be typical for Alloy 718. In anothernon-limiting embodiment, after a δ-phase heat treatment, the nickel-basealloy may include about 10 percent to about 25 percent by weight ofδ-phase precipitated at the grain boundaries. Such a result is observedto be typical for ATI 718Plus® alloy. It is understood that the amountof δ-phase, γ′-phase, and γ″-phase formed during a δ-phase heattreatment according to the present disclosure depends to some degree onthe specific composition of the nickel-base alloy, and the amount ofsuch phases formed may be determined readily and without undueexperimentation by those having ordinary skill.

In a non-limiting embodiment, the heat treating temperature is in a heattreating temperature range having a lower limit of 20° F. (11° C.)greater than the nose of the Time-Temperature-Transformation diagram(“TTT diagram”) for δ-phase precipitation for the specific nickel-basealloy, to an upper limit that is 100° F. (55.6° C.) below the nose forδ-phase precipitation in the specific TTT diagram. A TTT diagram for aparticular nickel-base alloy is a plot of temperature as a function ofthe logarithm of time for the alloy. TTT diagrams are used to determinewhen second phase transformations, such as δ-phase, γ′-phase, andγ″-phase transformations, begin and end during an isothermal heattreatment for a previously solution treated nickel-base alloy. A personskilled in the art understands that a particular TTT diagram is specificto a particular alloy composition. A TTT diagram for an embodiment ofAlloy 718 is reproduced in FIG. 5A, and a TTT diagram for ATI 718Plus®alloy is reproduced in FIG. 5B. The curve for δ-phase precipitation inthese TTT diagrams is labeled “δ (GB)” in FIG. 5A and “δ (Grain)” inFIG. 5B. As is understood by one having ordinary skill in the art, the“nose” of the δ-phase curve is known to a person of ordinary skill asbeing the portion of the δ-phase curve that is plotted to the earliestpoint in time on the time axis. For example the nose of δ-phase curve inFIG. 5A occurs at about 0.045 hours and about 900° C. The nose of theδ-phase curve in FIG. 5B occurs at about 0.035 hours and about 900° C.The curves shown in FIG. 5A and FIG. 5B are reproduced from Xie, et al.,“TTT Diagram of a Newly Developed Nickel-Base Superalloy—Allvac718Plus®, Proceedings: Superalloys 718, 625, 706 and Derivatives 2005,TMS (2005) pp. 193-202, which is hereby incorporated herein byreference. A person ordinarily skilled in the art is able to interpretand use TTT diagrams and, therefore, no further discussion concerningthe use of TTT diagrams is needed herein. In addition, TTT diagrams forspecific nickel-base alloys are publicly available or can be generatedby a person having ordinary skill in the art without undueexperimentation.

Referring to the schematic heat treating temperature-time profile (40)shown in FIG. 6, and with reference to the method steps generally shownin FIG. 4, a non-limiting embodiment of a method for heat treating a718-type nickel-base alloy according to the present disclosure comprisesheating (32) a 718-type nickel-base alloy to a heat treating temperaturein a heat treating temperature range of 1700° F. (926.7° C.) to 1725° F.(940.6° C.). In a non-limiting embodiment of method, the heated 718-typenickel-base alloy is held (34) within the heat treating temperaturerange for a heat treating time of from 30 minutes to 300 minutes. Afterholding (34) at the heat treating temperature for the heat treatingtime, the 718-type nickel-base alloy is air cooled and retains δ-phaseprecipitates at the grain boundaries. According to embodiments ofδ-phase heat treating method disclosed herein, the δ-phase grainboundary precipitates are primarily formed during the heating (32) andholding (34) steps. For this reason, the heating (32) and holding (34)steps may be collectively referred to as “δ-phase aging”.

In a non-limiting embodiment, after holding the nickel-base alloy at theheat treating temperature for the heat treating time, the nickel-basealloy is air cooled from the heat treating temperature to ambienttemperature. In a specific non-limiting embodiment, the nickel-basealloy is cooled at a cooling rate no greater than 1° F. per minute(0.56° C. per minute). Slow cooling is advantageous in certainnon-limiting embodiments according to the present disclosure becausesome γ′-phase precipitation is possible in a nickel-base alloy. Thesmall amount of γ′-phase that may precipitate during slow cooling willgenerally be coarse in structure and, therefore, have greater stabilitywith respect to thermal cycling and less impact on the mechanicalproperties of the alloy. It is preferred to have small amounts ofrelatively stable γ′-phase precipitate during slow cooling than to haveuncontrolled precipitation of γ′-phase during in-service thermalcycling.

Alloys processed according to any of the methods disclosed herein may beformed into mill products or other articles of manufacture. In certainnon-limiting embodiments according to the present disclosure, a 718-typenickel-base alloy is processed into an article of manufacture selectedfrom a foil, a honeycomb core, a face sheet, and a honeycomb panel by amethod including an embodiment of a method disclosed herein. As usedherein, the term “foil” refers to a sheet having a thickness less than0.006 inch (0.15 mm) and any width and length. As a practical matter,the width of a foil is limited by the capacity of cold rolling equipmentused to roll the alloy. In certain non-limiting embodiments of methodsaccording to the present disclosure, alloys processed according toembodiments of the method disclosed herein may be processed into foilshaving a width up to 18 inches (0.46 m), up to 24 inches (0.61 m), or upto 36 inches (0.91 m).

For applications in which the maximum in-service temperature to which analloy will be subjected is known and is about 1700° F. (926.7° C.) orless, non-limiting embodiments of a method according to the presentdisclosure may further include a stabilizing heat treatment subsequentto the step of cooling the nickel-base alloy from the heat treatingtemperature. In a non-limiting embodiment according to the presentdisclosure, the stabilizing heat treatment comprises heating thenickel-base alloy to a stabilizing heat treating temperature and holdingthe alloy at the temperature for at least 2 hours, or for at least 2hours up to 4 hours. In non-limiting embodiments, the stabilizing heattreating temperature is the maximum in-service temperature to which thealloy will be subjected and is in a range of 1700° F. (926.7° C.) orless, or in a range of 1700° F. (926.7° C.) to 1450° F. (787.8° C.). Asused herein, the term “maximum in-service temperature” refers to themaximum temperature that the particular nickel-base alloy is expected toexperience when the alloy or an article including the alloy is used forits intended purpose. Subsequent to a stabilizing heat treatmentaccording the present disclosure, the nickel-base alloy is air cooledfrom the stabilizing heat treating temperature to ambient temperature.In another non-limiting embodiment, the nickel-base alloy is cooled at acooling rate no greater than 1° F. per minute (0.56° C. per minute) fromthe stabilizing heat treating temperature to ambient temperature.

It is recognized that non-limiting embodiments of the δ-phase heattreatment and δ-phase aging according to this disclosure could be usedon any form or shape of nickel-base alloy or 718-type nickel-base alloy.Various forms include commercial mill products such as, but not limitedto, bar, rod, plate, sheet, strip, and extrusion. It is furtherrecognized that non-limiting embodiments of the δ-phase heat treatmentand δ-phase aging according to this disclosure also could be used onmanufactured products such as, but not limited to, formed products,joined products, and the like comprising nickel-base alloys or 718-typenickel-base alloys.

In a non-limiting embodiment of a method of heat treating a nickel-basealloy according to the present disclosure, the nickel-base alloycomprises or consists of, in percent by weight: 17.00 to 21.00 chromium;2.50 to 3.10 molybdenum; 5.20 to 5.80 niobium; 0.50 to 1.00 titanium;1.20 to 1.70 aluminum; 8.00 to 10.00 cobalt; 8.00 to 10.00 iron; 0.008to 1.40 tungsten; 0.003 to 0.008 boron; 0.01 to 0.05 carbon; up to 0.35manganese; up to 0.035 silicon; 0.004 to 0.020 phosphorus; up to 0.025sulfur; nickel; and incidental impurities. Such non-limiting embodimentfurther comprises: heat treating the nickel-base alloy to a heattreating temperature in a range of 1700° F. (926.7° C.) to 1725° F.(940.6° C.); holding the nickel-base alloy at the heat treatingtemperature for a heat treating time in a range of 30 minutes to 300minutes that is sufficient to form an equilibrium or near-equilibriumconcentration of δ-phase grain boundary precipitates within thenickel-base alloy and up to 25 percent by weight of total γ′-phase andγ″-phase within the alloy; and air cooling the nickel-base alloy. Innon-limiting embodiments, the nickel-base alloy comprises one of a foil,a honeycomb core, a face sheet, and a honeycomb panel.

A non-limiting aspect according to the present disclosure is directed toa 718-type nickel-base alloy, as that term is defined herein, and thatcomprises an austenite matrix comprising grain boundaries. Anequilibrium or near-equilibrium concentration of δ-phase precipitates ispresent at the grain boundaries, and up to 25 percent by weight of totalγ′-phase and γ″-phase is present in the alloy.

One specific non-limiting example of a 718-type nickel-base alloyaccording to the present disclosure comprises an austenite matrixincluding grain boundaries, an equilibrium or near-equilibriumconcentration of δ-phase precipitates at the grain boundaries, up to 25percent by weight of total γ′-phase and γ″-phase precipitates, and up to14 weight percent iron. Another specific non-limiting example of a718-type nickel-base alloy according to the present disclosure comprisesan austenite matrix including grain boundaries, an equilibrium ornear-equilibrium concentration of δ-phase precipitates at the grainboundaries, up to 25 percent by weight of total γ′-phase and γ″-phaseprecipitates, and 6 weight percent up to 14 weight percent iron.

Another specific, non-limiting example of a 718-type nickel-base alloyaccording to the present disclosure comprises an austenite matrixincluding grain boundaries, a near-equilibrium concentration of δ-phaseprecipitates at the grain boundaries, and up to 25 percent by weight oftotal γ′-phase and γ″-phase precipitates. The alloy further comprises orconsists of, in percent by weight: up to 0.1 carbon; 12 to 20 chromium;up to 4 molybdenum; up to 6 tungsten; 5 to 12 cobalt; 6 to 14 iron; 4 to8 niobium; 0.6 to 2.6 aluminum; 0.4 to 1.4 titanium; 0.003 to 0.03phosphorus; 0.003 to 0.015 boron; nickel; and incidental impurities;wherein a sum of the weight percent of molybdenum and the weight percentof tungsten is at least 2 and not more than 8; a sum of atomic percentaluminum and atomic percent titanium is from 2 to 6; a ratio of atomicpercent aluminum to atomic percent titanium is at least 1.5; and the sumof atomic percent aluminum and atomic percent titanium divided by atomicpercent niobium is from 0.8 to 1.3.

Yet another specific, non-limiting example of a 718-type nickel-basealloy according to the present disclosure comprises an austenite matrixincluding grain boundaries, a near-equilibrium concentration of δ-phaseprecipitates at the grain boundaries, and up to 25 percent by weight oftotal γ′-phase and γ″-phase precipitates. The alloy further comprises orconsists of, in percent by weight: 0 to about 0.08 carbon; 0 to about0.35 manganese; about 0.003 to about 0.03 phosphorous; 0 to about 0.015sulfur; 0 to about 0.35 silicon; about 17 to about 21 chromium; about 50to about 55 nickel; about 2.8 to about 3.3 molybdenum; about 4.7 toabout 5.5 niobium; 0 to about 1 cobalt; 0.003 to about 0.015 boron; 0 toabout 0.3 copper; and balance iron (typically about 12 to about 20percent), aluminum, titanium, and incidental impurities; wherein the sumof atomic percent aluminum and atomic percent titanium is from about 2to about 6 percent; the ratio of atomic percent aluminum to atomicpercent titanium is at least about 1.5; and the sum of atomic percent ofaluminum plus atomic percent titanium divided by atomic percent niobiumequals from about 0.8 to about 1.3.

A further specific non-limiting example of a 718-type nickel-base alloyaccording to the present disclosure comprises an austenite matrixcomprising grain boundaries, a near-equilibrium concentration of δ-phaseprecipitates at the grain boundaries, and up to 25 percent by weight oftotal γ′-phase and γ″-phase precipitates. The alloy further comprises orconsists of, in percent by weight: 0.01 to 0.05 carbon; up to 0.35manganese; up to 0.035 silicon; 0.004 to 0.020 phosphorus; up to 0.025sulfur; 17.00 to 21.00 chromium; 2.50 to 3.10 molybdenum; 5.20 up to5.80 niobium; 0.50 up to 1.00 titanium; 1.20 to 1.70 aluminum; 8.00 to10.00 cobalt; 8.00 to 10.00 iron; 0.008 to 1.40 tungsten; 0.003 to 0.008boron; nickel; and incidental impurities.

Still a further non-limiting example of a 718-type nickel-base alloyaccording to the present disclosure comprises an austenite matrixcomprising grain boundaries, a near-equilibrium concentration of δ-phaseprecipitates at the grain boundaries, and up to 25 percent by weight oftotal γ′-phase and γ″-phase precipitates. The alloy further comprises orconsists of, in percent by weight: 50.0 to 55.0 nickel; from 17 to 21.0chromium; up to 0.08 carbon; up to 0.35 manganese; up to 0.35 silicon;from 2.8 to 3.3 molybdenum; greater than 0 up to 5.5 niobium andtantalum, wherein the sum of niobium and tantalum is from 4.75 to 5.5;from 0.65 to 1.15 titanium; from 0.20 to 0.8 aluminum; up to 0.006boron; iron; and incidental impurities.

An aspect of this disclosure includes an article of manufacturefabricated according to a method of this disclosure and/or including analloy according to this disclosure. Non-limiting examples of articles ofmanufacture according to this disclosure include a face sheet, ahoneycomb core, and a honeycomb panel of a TPS for a hypersonic flightvehicle or a space vehicle.

The examples that follow are intended to further describe certainnon-limiting embodiments, without restricting the scope of the presentinvention. Persons having ordinary skill in the art will appreciate thatvariations of the following examples are possible within the scope ofthe invention, which is defined solely by the claims.

Example 1

A sheet of a 0.080 inch (2.03 mm) thick ATI 718Plus® alloy and a 0.4inch (10.2 mm) diameter rod of Alloy 718 were heat treated according toa non-limiting embodiment of the present disclosure by heating the twoalloys to 1725° F. (940.6° C.) and holding at temperature for 3 hours.The samples were then air cooled.

For comparison purposes, samples of the same alloys were heat treatedaccording to the following standard γ′/γ″ aging heat treatments.

A 0.080 inch (2.03 mm) thick sheet of ATI 718Plus® alloy was heated to1750° F. (954.4° C.), held at temperature for 45 minutes, and aircooled. After cooling, the sample was aged at 1450° F. (787.8° C.) for 8hours. The sample was cooled at 100° F./h (55.6° C./h) to 1300° F.(704.4° C.), and held at 1300° F. (704.4° C.) for 8 hours. After aging,the ATI 718Plus® alloy sample was air cooled.

In addition, a 0.4 inch (10.2 mm) diameter rod of Alloy 718 was heatedto 1750° F. (954.4° C.), held at temperature for 45 minutes, and aircooled. After cooling, the Alloy 718 sample was aged at 1325° F. (718.3°C.) for 8 hours. The sample was cooled at 100° F./h (55.6° C./h) to1150° F. (621.1° C.), and held at 1150° F. (621.1° C.) for 8 hours.After aging, the sample was air cooled.

Example 2

The heat treated samples from Example 1 were subjected to thermalcycling. The ATI 718Plus® alloy samples were cycled from ambienttemperature to either 1650° F.). (898.9° or 1550° F. (843.3° C.). TheAlloy 718 samples were cycled from ambient temperature to 1650° F.).(898.9°. FIG. 7 is a schematic representation of the thermal cyclesused, wherein the indicated temperatures are of the alloy samples,rather than the furnace temperature. The top plot included in FIG. 7reflects a slower alloy cooling rate (about 10° F./min (5.6° C./min))and represents the general behavior of thicker samples. The bottom plotreflects a faster cooling rate (about 1500° F./min (833° C./min)) andrepresents the general behavior of thinner samples. The cooling ratesdepicted in FIG. 7 are estimated, but the peak temperatures and holdtimes in FIG. 7 accurately represent what the alloys experienced.

Example 3

After exposure to thermal cycling, the samples were tensile tested atroom temperature according to standard test procedures described in ASTME8-09/E8M-09. Plots of ultimate tensile strengths of as-heat treatedsamples and after 1 and 5 thermal cycles are provided in FIG. 8. Theplots on the left side of FIG. 8 show ultimate tensile strengths as afunction of the number of thermal treatment cycles for ATI 718Plus®alloy samples that was cooled at the slower cooling rate discussed inExample 2. The plots on the right side of FIG. 8 show ultimate tensilestrengths as a function of the number of thermal treatment cycles forATI 718Plus® alloy that was cooled at the faster cooling rate discussedin Example 2. The top row of plots in FIG. 8 are for ATI 718Plus® alloythat was heat treated according to embodiments of the present disclosureas described in Example 1, thermally cycled to a peak sample temperatureof 1650° F. (898.9° C.). The bottom row of plots in FIG. 8 are for ATI718Plus® alloy that was heat treated according to embodiments of thepresent disclosure as described in Example 1, thermally cycled to a peaksample temperature of 1550° F. (843.3° C.).

Examination of FIG. 8 shows that the inventive δ-phase aging treatmentsmay provide lower initial strengths than conventional γ′/γ″ agingtreatments, but there is significantly less variability of ultimatetensile strengths during thermal cycling. This is more evident in FIG.9, which displays the data of FIG. 8 but wherein the y-axis representsthe ratio of ultimate tensile strength after the sample was subjected tothermal cycling to the ultimate tensile strength in the as-heat treatedcondition. FIG. 9 clearly shows that the δ-phase heat treatmentembodiment according to the present disclosure produced an alloyexhibiting a significantly more stable ultimate tensile strength afterthermal cycling for at least 5 thermal cycles.

FIG. 10 includes plots of yield strengths for the samples included inFIG. 8. The plots of FIG. 10 are in the same orientations as in FIG. 8with regard to cooling rates and peak sample temperatures. Consideringwhat is shown in FIG. 10, it will be seen that the inventive δ-phaseaging treatments may provide lower initial yield strengths thanconventional γ′/γ″ aging treatments, but with significantly lessvariability of yield strengths for the δ-phase heat treated alloysduring thermal cycling. This is more evident in FIG. 11, which displaysthe data of FIG. 10 but wherein the y-axis represents the ratio of yieldstrength after the sample was subjected to thermal cycling to the yieldstrength in the as-heat treated condition. FIG. 11 clearly shows thatthe δ-phase heat treatment embodiment according to the presentdisclosure produced an alloy exhibiting significantly more stable yieldstrength after thermal cycling for at least 5 thermal cycles.

FIG. 12 includes plots of percent elongation for the samples included inFIG. 8. The plots of FIG. 12 are in the same orientations as in FIG. 8with regard to cooling rates and peak sample temperatures. Consideringwhat is shown in FIG. 12, it will be seen that the inventive δ-phaseaging treatments may provide higher percent elongation than conventionalγ′/γ″ aging treatments, but with significantly less variability ofpercent elongation for the δ-phase heat treated alloys during thermalcycling. This is more evident in FIG. 13, which displays the data ofFIG. 12 but wherein the y-axis represents the ratio of percentelongation after the sample was subjected to thermal cycling to thepercent elongation in the as-heat treated condition. FIG. 13 clearlyshows that the δ-phase heat treatment embodiment according to thepresent disclosure produced an alloy exhibiting a significantly morestable percent elongation after thermal cycling for at least 5 thermalcycles.

Samples of Alloy 718 as heat treated in Example 1 and as thermallycycled to 1650° F.). (898.9° in Example 2 were tensile tested at roomtemperature according to standard test procedures described in ASTME8-09/E8M-09. Plots of ultimate tensile strengths of as-heat treatedsamples and for samples after 1 and 5 thermal cycles are plotted in FIG.14. The plots on the left side of FIG. 14 show ultimate tensilestrengths as a function of the number of thermal treatment cycles forAlloy 718 alloy that was thermally cycled using the slower cooling ratedescribed in Example 2, and the plots on the right were thermally cycledusing the faster cooling rate described in Example 2.

Examination of FIG. 14 shows that the inventive δ-phase aging treatmentsmay provide an alloy exhibiting lower initial strengths thanconventional γ′/γ″ aging treatments, but also exhibiting significantlyless variability in ultimate tensile strength when subjected to thermalcycling. This is more evident in FIG. 15, which displays the data ofFIG. 14 but wherein the y-axis represents the ratio of ultimate tensilestrength after the sample was subjected to thermal cycling to theultimate tensile strength in the as-heat treated condition. FIG. 15clearly shows that the δ-phase heat treatment embodiment according tothe present disclosure produced an alloy exhibiting a significantly morestable ultimate tensile strength after thermal cycling for at least 5thermal cycles

FIG. 16 includes plots of yield strengths for the samples included inFIG. 14. The plots of FIG. 16 are in the same orientations as in FIG. 14with regard to cooling rates and peak sample temperatures. Consideringwhat is shown in FIG. 16, it will be seen that the inventive δ-phaseaging treatments may provide lower initial yield strengths thanconventional γ′/γ″ aging treatments, but with significantly lessvariability of yield strengths for the δ-phase heat treated alloysduring thermal cycling. This is more evident in FIG. 17, which displaysthe data of FIG. 16 but wherein the y-axis represents the ratio of yieldstrength after the sample was subjected to thermal cycling to the yieldstrength in the as-heat treated condition. FIG. 17 clearly shows thatthe δ-phase heat treatment embodiment according to the presentdisclosure produced an alloy exhibiting significantly more stable yieldstrength after thermal cycling for at least 5 thermal cycles.

FIG. 18 includes plots of percent elongation for the samples included inFIG. 14. The plots of FIG. 18 are in the same orientations as in FIG. 14with regard to cooling rates and peak sample temperatures. Consideringwhat is shown in FIG. 18, it will be seen that the inventive δ-phaseaging treatments may provide higher percent elongation than conventionalγ′/γ″ aging treatments, but with significantly less variability ofpercent elongation for the δ-phase heat treated alloys during thermalcycling. This is more evident in FIG. 19, which displays the data ofFIG. 18 but wherein the y-axis represents the ratio of percentelongation after the sample was subjected to thermal cycling to thepercent elongation in the as-heat treated condition. FIG. 19 clearlyshows that the δ-phase heat treatment embodiment according to thepresent disclosure produced an alloy exhibiting a significantly morestable percent elongation after thermal cycling for at least 5 thermalcycles.

Example 4

Surface regions of the samples tensile tested in Example 3 were examinedusing dark field optical microscopy. FIG. 20A is a photomicrograph of asurface region of an ATI 718Plus® alloy sample that was δ-phase heattreated as described in Example 1. The thicker white platelets primarilydisposed on grain boundaries in FIG. 20A are δ-phase platelets thatresult from the δ-phase heat treatment according to non-limitingembodiments of the present disclosure. FIG. 20B is a photomicrograph ofa surface region of the same ATI 718Plus® alloy sample after beingsubjected to 5 thermal cycles to a peak sample temperature of 1650° F.).(898.9°. It may be seen that there is little, if any, difference in theamount of δ-phase platelets in the samples after 5 thermal cycles to1650° F.). (898.9° peak sample temperature. This correlates well withthe tensile tests of Example 3 showing that ATI 718Plus® alloy samplesδ-phase heat treated as described in Example 1 exhibited a lowervariability of tensile properties on thermal cycling.

FIG. 20C is a photomicrograph of a surface region of an ATI 718Plus®alloy sample that was heat treated according to the conventional γ′/γ″heat treatment described in Example 1. It is observed that themicrostructure includes a small amount of δ-phase grain boundaryprecipitates and that the amount is less than in the samples subjectedto the δ-phase heat treatment, as seen in FIG. 20A. However, it is seenin FIG. 20D that after 5 thermal cycles to 1650° F.). (898.9°, themicrostructure has clearly changed to include a significant amount ofδ-phase at the grain boundaries. This change in microstructure resultingfrom thermal cycling is reflected in the deterioration in the tensileproperties of γ′/γ″ heat treated and thermally cycled nickel-basesuperalloy samples presented in Example 3.

FIG. 21A is a photomicrograph of a surface region of an ATI 718Plus®alloy sample that was δ-phase heat treated as described in Example 1.The thicker white plates primarily on the grain boundaries are δ-phaseplatelets that result from the δ-phase heat treatment according tonon-limiting embodiments of the present disclosure. FIG. 21B is aphotomicrograph of a surface region of the same sample after thermalcycles to a peak sample temperature of 1550° F. (843.3° C.). It may beobserved that there is little, if any, difference in the amount ofδ-phase platelets after 5 thermal cycles to the 1550° F. (843.3° C.)peak sample temperature. This correlates well with the tensile tests ofExample 3, showing that ATI 718Plus® alloy samples δ-phase heat treatedas described in Example 1 exhibited a lower variability of tensileproperties on thermal cycling.

FIG. 21C is a photomicrograph of a surface region of an ATI 718Plus®alloy sample that was heat treated according to the conventional γ′/γ″heat treatment described in Example 1. It is observed that themicrostructure may include a small amount of δ-phase grain boundaryprecipitates, and that the amount is less than in the samples subjectedto the δ-phase heat treatment, as seen in FIG. 21A. However, it is seenin FIG. 21D that after 5 thermal cycles to 1550° F. (843.3° C.), themicrostructure has clearly changed to include a significant amount ofδ-phase at the grain boundaries. This change in microstructure resultingfrom thermal cycling is reflected in the deterioration in tensileproperties of γ′/γ″ heat treated and thermally cycled nickel-basesuperalloy samples presented in Example 3.

FIG. 22A is a photomicrograph of a surface region of an Alloy 718 samplethat was δ-phase heat treated as described in Example 1. The thickerwhite plates that are primarily on grain boundaries are δ-phaseplatelets that result from the δ-phase heat treatment according tonon-limiting embodiments of the present disclosure.

FIG. 22B is a photomicrograph of a surface region of the same sampleafter 5 thermal cycles to a peak sample temperature of 1650° F.).(898.9°. It is observed that there is little, if any, difference in theamount of δ-phase platelets after 5 thermal cycles to 1650° F.). (898.9°peak sample temperature. This correlates well with the tensile tests ofExample 3, showing that Alloy 718 samples δ-phase heat treated asdescribed in Example 1 exhibited a lower variability of tensileproperties on thermal cycling.

FIG. 22C is a photomicrograph of a surface region of an Alloy 718 samplethat was heat treated according to a conventional γ′/γ″ heat treatmentdescribed in Example 1. It is observed that the microstructure mayinclude a small amount of δ-phase grain boundary precipitates, and thatthe amount is less than in the samples subjected to the δ-phase heattreatment, as seen in FIG. 22A. However, it is seen in FIG. 22D thatafter 5 thermal cycles to 1650° F.). (898.9°, the microstructure hasclearly changed to include a significant amount of δ-phase at the grainboundaries. This change in microstructure resulting from thermal cyclingis reflected in the deterioration of tensile properties of γ′/γ″ heattreated and thermally cycled nickel-base superalloys presented inExample 3.

The present disclosure has been written with reference to variousexemplary, illustrative, and non-limiting embodiments. It will berecognized by persons having ordinary skill in the art that varioussubstitutions, modifications, or combinations of any of the disclosedembodiments (or portions thereof) may be made without departing from thescope of the invention, as defined solely by the claims. Thus, it iscontemplated and understood that the present disclosure embracesadditional embodiments not expressly set forth herein. This disclosureis not limited by the description of the various exemplary,illustrative, and non-limiting embodiments, but rather solely by theclaims. In this manner, it will be understood that the claims may beamended during prosecution of the present patent application to addfeatures to the claimed invention as variously described herein.

1. A method for heat treating a nickel-base alloy, comprising: heating a nickel-base alloy to a heat treating temperature in a heat treating temperature range, wherein the nickel-base alloy comprises nickel, chromium, and iron, is strengthened by niobium, and optionally comprises one or more of aluminum and titanium alloying additions; holding the nickel-base alloy within the heat treating temperature range for a heat treating time sufficient to form an equilibrium or near-equilibrium concentration of δ-phase precipitates in grain boundaries of the alloy and up to 25 percent by weight of total γ′-phase and γ″-phase within the alloy; and cooling the nickel-base alloy.
 2. The method of claim 1, wherein the heat treating temperature range is in the range of a temperature that is 20° F. (11° C.) greater than the nose of the TTT diagram for delta phase precipitation to a temperature that is 100° F. (55.6° C.) below the nose of the TTT diagram.
 3. The method of claim 1, wherein the heat treating time is in a range of 30 minutes to 300 minutes.
 4. The method of claim 1, wherein cooling the nickel-base alloy comprises air cooling.
 5. The method of claim 1, wherein cooling the nickel-base alloy comprises cooling the alloy at a cooling rate no greater than about 1° F. per minute (0.56° C. per minute).
 6. (canceled)
 7. The method of claim 1, wherein the nickel-base alloy comprises, in percent by weight: up to 0.1 carbon; 12 to 20 chromium; up to 4 molybdenum; up to 6 tungsten; 5 to 12 cobalt; 6 to 14 iron; 4 to 8 niobium; 0.6 to 2.6 aluminum; 0.4 to 1.4 titanium; 0.003 to 0.03 phosphorus; 0.003 to 0.015 boron; nickel; and incidental impurities; wherein a sum of the weight percent of molybdenum and the weight percent of tungsten is at least 2 and not more than 8; a sum of atomic percent aluminum and atomic percent titanium is from 2 to 6; a ratio of atomic percent aluminum to atomic percent titanium is at least 1.5; and the sum of atomic percent aluminum and atomic percent titanium divided by atomic percent niobium is from 0.8 to 1.3.
 8. The method of claim 1, wherein the nickel-base alloy comprises, in percent by weight: 0 to about 0.08 carbon; 0 to about 0.35 manganese; about 0.003 to about 0.03 phosphorous; 0 to about 0.015 sulfur; 0 to about 0.35 silicon; about 17 to about 21 chromium; about 50 to about 55 nickel; about 2.8 to about 3.3 molybdenum; about 4.7 percent to about 5.5 niobium; 0 to about 1 cobalt; about 0.003 to about 0.015 boron; 0 to about 0.3 copper; 12 to 20 iron; aluminum; titanium; and incidental impurities; wherein the sum of atomic percent aluminum and atomic percent titanium is from about 2 to about 6 percent; the ratio of atomic percent aluminum to atomic percent titanium is at least about 1.5; and the sum of atomic percent of aluminum plus titanium divided by atomic percent of niobium equals from about 0.8 to about 1.3.
 9. The method of claim 1, wherein the nickel-base alloy comprises, in percent by weight: 0.01 to 0.05 carbon; up to 0.35 manganese; up to 0.035 silicon; 0.004 to 0.020 phosphorus; up to 0.025 sulfur; 17.00 to 21.00 chromium; 2.50 to 3.10 molybdenum; 5.20 to 5.80 niobium; 0.50 to 1.00 titanium; 1.20 to 1.70 aluminum; 8.00 to 10.00 cobalt; 8.00 to 10.00 iron; 0.008 to 1.40 tungsten; 0.003 to 0.008 boron; nickel; and incidental impurities.
 10. The method of claim 1, wherein the nickel-base alloy comprises, in percent by weight: 50.0 to 55.0 nickel; 17 to 21.0 chromium; up to 0.08 carbon; up to 0.35 manganese; up to 0.35 silicon; 2.8 to 3.3 molybdenum; greater than 0 to 5.5 niobium and tantalum, wherein the sum of niobium and tantalum is from 4.75 to 5.5; 0.65 to 1.15 titanium; 0.20 to 0.8 aluminum; up to 0.006 boron; iron; and incidental impurities.
 11. The method of claim 1, wherein the nickel-base alloy comprises at least one of a foil, a honeycomb core, and a honeycomb panel.
 12. The method of claim 1, further comprising, subsequent to cooling the nickel-base alloy, stabilizing heat treating the nickel-base alloy, wherein stabilizing heat treating comprises: heating the nickel-base alloy to a stabilizing heat treating temperature of 1700° F. (926.7° C.) or less, wherein the heat treating temperature is equivalent to an expected maximum in-service temperature of an article comprising the nickel-base alloy; and cooling the nickel-base alloy from the stabilizing heat treating temperature.
 13. The method of claim 12, wherein cooling the nickel-base alloy from the stabilizing heat treating temperature comprises air cooling.
 14. The method of claim 12, wherein cooling the nickel-base alloy from the stabilizing heat treating temperature comprises cooling at a cooling rate no greater than about 1° F. per minute (0.56° C. per minute).
 15. A method of heat treating a nickel-base alloy, comprising: heating a nickel-base alloy to a heat treating temperature in a range of 1700° F. (926.7° C.) to 1725° F. (940.6° C.); holding the nickel-base alloy at the heat treating temperature for a heat treating time in a range of 30 minutes to 300 minutes; and air cooling the nickel-base alloy; wherein the nickel-base alloy comprises, in percent by weight, 17.00 to 21.00 chromium, 2.50 to 3.10 molybdenum, 5.20 to 5.80 niobium, 0.50 to 1.00 titanium, 1.20 to 1.70 aluminum, 8.00 to 10.00 cobalt, 8.00 to 10.00 iron, 0.008 to 1.40 tungsten, 0.003 to 0.008 boron, 0.01 to 0.05 carbon, up to 0.35 manganese, up to 0.035 silicon, 0.004 to 0.020 phosphorus, up to 0.025 sulfur, nickel, and incidental impurities.
 16. The method of claim 15, further comprising, after cooling the nickel-base alloy, stabilizing heat treating the nickel-base alloy, wherein stabilizing heat treating comprises: heating the nickel-base alloy to a stabilizing heat treating temperature that is an expected maximum in-service temperature of an article comprising the nickel-base alloy and is about 1700° F. (926.7° C.) or less; and air cooling the nickel-base alloy.
 17. The method of claim 15, wherein the nickel-base alloy comprises at least one of a foil, a honeycomb core, and a honeycomb panel.
 18. A 718-type-nickel-base alloy comprising: an austenite matrix including grain boundaries; an equilibrium or near-equilibrium concentration of δ-phase precipitates at the grain boundaries; and up to 25 percent by weight of total γ′-phase and γ″-phase precipitates; and wherein the 718-type nickel-base alloy comprises nickel, chromium, and iron, and is strengthened by niobium, and optionally one or more of aluminum and titanium alloying additions.
 19. The 718-type nickel-base alloy of claim 18, comprising in percent by weight, up to 0.1 carbon, 12 to 20 chromium, up to 4 molybdenum, up to 6 tungsten, 5 to 12 cobalt, 6 up to 14 iron, 4 to 8 niobium, 0.6 to 2.6 aluminum, 0.4 to 1.4 titanium, 0.003 to 0.03 phosphorus, 0.003 to 0.015 boron, nickel, and incidental impurities; wherein a sum of the weight percent of molybdenum and the weight percent of tungsten is at least 2 and not more than 8; wherein a sum of atomic percent aluminum and atomic percent titanium is from 2 to 6; wherein a ratio of atomic percent aluminum to atomic percent titanium is at least 1.5; and wherein a sum of atomic percent aluminum and atomic percent titanium divided by atomic percent niobium is from 0.8 to 1.3.
 20. The 718-type nickel-base alloy of claim 18, comprising, in percent by weight: 0 to about 0.08 carbon, 0 to about 0.35 manganese; about 0.003 to about 0.03 phosphorous; 0 to about 0.015 sulfur; 0 to about 0.35 silicon; about 17 to about 21 chromium; about 50 to about 55 nickel; about 2.8 up to about 3.3 molybdenum; about 4.7 to about 5.5 niobium; 0 to about 1 cobalt; about 0.003 to about 0.015 boron; 0 to about 0.3 copper; 12 to 20 iron; aluminum; titanium; and incidental impurities; wherein the sum of atomic percent aluminum and atomic percent titanium is from about 2 to about 6 percent; the ratio of atomic percent aluminum to atomic percent titanium is at least about 1.5; and the sum of atomic percent of aluminum plus titanium divided by atomic percent of niobium equals from about 0.8 to about 1.3.
 21. The 718-type nickel-base alloy of claim 18, comprising, in percent by weight: 0.01 to 0.05 carbon; up to 0.35 manganese; up to 0.035 silicon; 0.004 to 0.020 phosphorus; up to 0.025 sulfur; 17.00 to 21.00 chromium; 2.50 to 3.10 molybdenum; 5.20 to 5.80 niobium; 0.50 to 1.00 titanium; 1.20 to 1.70 aluminum; 8.00 to 10.00 cobalt; 8.00 to 10.00 iron; 0.008 to 1.40 tungsten; 0.003 to 0.008 boron; nickel; and incidental impurities.
 22. The 718-type nickel-base alloy of claim 18, comprising, in percent by weight: 50.0 to 55.0 nickel; 17 to 21.0 chromium; up to 0.08 carbon; up to 0.35 manganese; up to 0.35 silicon; 2.8 to 3.3 molybdenum; greater than 0 to 5.5 niobium and tantalum, wherein the sum of niobium and tantalum is from 4.75 to 5.5; 0.65 to 1.15 titanium; 0.20 to 0.8 aluminum; up to 0.006 boron; iron; and incidental impurities.
 23. An article of manufacture made by a process comprising the method of claim
 1. 24. The article of manufacture of claim 23, wherein the article of manufacture comprises at least one of a face sheet, a honeycomb core, and a honeycomb panel of a thermal protection system for a hypersonic flight vehicle or a space vehicle.
 25. An article of manufacture comprising an alloy according to claim
 12. 26. The article of manufacture of claim 25 comprising one of a face sheet, a honeycomb core, and a honeycomb panel of a thermal protection system for a hypersonic flight vehicle. 